The present invention relates to a household apparatus for steam generation comprising a water reservoir at atmospheric pressure, a boiler for vaporising the water, means for feeding the water from the reservoir to the boiler, and a steam delivery duct from the boiler to a steam user appliance.
The present invention also relates to a heating unit comprising a heating source and a temperature sensor, suitable to be used in a boiler of said household apparatus.
Household apparatuses for steam generation are known.
Typically, said household apparatuses comprise a heating source for vaporising the water of the boiler, and means for maintaining a desired level of pression and a desired level of water into the boiler.
Document DE 37 20 583 describes an apparatus for steam generation comprising a boiler for vaporising the water, a pump for feeding water to the boiler, a heating source helically wound around the boiler, two temperature sensors also helically wound around the boiler, a manometer and a pressure regulator. One of the two sensors is used for detecting the temperature of the heating source and for recalling water into the boiler when the detected temperature exceeds a first threshold temperature. The second sensor is used for detecting the temperature of the heating source and for switching it off when the detected temperature exceeds a second threshold temperature which is higher than the first threshold temperature. On the other hand, the manometer and the pressure regulator are used to maintain a desired value of the steam pressure into the boiler.
Document DE 43 04 532 describes an apparatus for steam generation comprising a boiler for vaporising the water and a pump for feeding water to the boiler. In turn, the boiler comprises a heating source having an elevated portion and a temperature sensor arranged in the proximity of said elevated portion of said heating source. In addition, the apparatus described also comprises a thermostat co-operating with said temperature sensor so as to maintain a desired level of water into the boiler. More in particular, when the temperature detected by the temperature sensor exceeds a certain threshold temperature, the thermostat switches the pump on so as to recall water into the boiler and restore the desired level of water into the boiler.
Document EP 0 877 200, filed by the Applicant, describes a household apparatus for steam generation comprising a water reservoir at atmospheric pressure, a boiler for vaporising the water, a pump for feeding the water from the reservoir to the boiler, and a steam-delivery duct from the boiler to a steam user appliance. In turn, the boiler comprises a U-shaped resistor and a temperature sensor arranged inside an outer support structure. The curved portion of the resistor raises above the remaining portion, and the outer support structure of the temperature sensor is welded onto said elevated curved portion in a transverse direction with respect to it.
The temperature sensor is suitable to detect the temperature of the resistor. When the water level into the boiler decreases due to steam delivery, the elevated portion of the resistor (which in normal operating conditions is immersed into the water) emerges from the water, the temperature sensor detects a rise of temperature and suitable control means switch the water feeding pump on so as to introduce into the boiler a quantity of water sufficient to cover again the elevated portion of the resistor.
This apparatus has the advantage that when the level of water decreases, only the elevated portion of the resistor emerges from the water, thus allowing the remaining portion of resistor to operate always immersed in the water, and thus preventing temperature rises that may be dangerous for its life.
Nevertheless, the inventors of the present invention have found that the apparatus described in this latest document
wherein the outer support structure of the temperature sensor is welded to the elevated portion of the heating source in transverse direction with respect to itxe2x80x94is not very reliable as a minimum error of positioning of the support structure on the elevated portion may cause a wrong positioning of the sensor with respect to the heating source.
In fact, for the apparatus to operate correctly, that portion of the outer support structure, in which the sensor is exactly located, must be welded onto the elevated portion of the heating source.
Therefore, the inventors of the present invention faced the problem of providing a more reliable household apparatus for steam generation.
Thus, in a first aspect thereof the present invention relates to a household apparatus for steam generation comprising
a water reservoir at atmospheric pressure;
a boiler comprising a heating unit in turn including
a heating source for vaporising the water suitable to be at least partly immersed in the water and having an elevated portion which extends along a predetermined direction, and
a temperature sensor contained into a protective sheath, said protective sheath being in contact with said heating source,
means for feeding the water from the reservoir to the boiler,
means for delivering the steam from the boiler to a steam user appliance, characterised in that the contact area between said protective sheath and said elevated portion extends along said predetermined direction so as to make said contact area relatively wide.
In the apparatus of the invention, the contact area between the protective sheath and the elevated portion of the heating source is relatively wide as it extends along the same direction in which the elevated portion extends. This allows making the positioning of the sensor with respect to the elevated portion more reliable.
Furthermore, the relatively wide contact area between the protective sheath and the elevated portion advantageously allows facilitating, during the assembly process of the apparatus of the invention, the positioning of the sensor inside the protective sheath, and of the protective sheath with respect to the elevated portion of the heating source. In fact, a relatively wide contact area allows increasing the tolerances of said positionings.
Furthermore, in the apparatus of the invention, thanks to the elevated portion, the remaining portion of the heating source substantially operates always immersed in the water. This advantageously allows preventing frequent rises of temperature of the entire heating source which may impair its good operation and life.
Advantageously, the contact area between said protective sheath and said elevated portion has an extension at least equal to 5 mm. Preferably, said extension is comprised between 5 and 30 mm. This allows having a good margin for the positioning of the protective sheath of the sensor with respect to the elevated portion.
Advantageously, said elevated portion extends in a substantially rectilinear way.
According to an alternative, said elevated portion substantially extends according to a circumference arch.
Typically, said heating source is substantially U-shaped, comprising two substantially rectilinear and parallel opposed portions and a curvilinear portion connecting the two rectilinear portions.
In this case, said elevated portion is preferably arranged in correspondence with one of the two rectilinear portions of said U-shape.
According to an alternative, said heating source can, for example, have a folded U-shape or a helical shape.
The dimensions of the heating source are advantageously selected in function of the desired power and of the dimensions of the boiler suitable to contain it.
Preferably, said protective sheath is welded along said elevated portion. More preferably, said protective sheath is welded along an upper portion of said elevated portion. In this way, the elevated portion of the heating source is prevented from emerging from the water before the temperature sensor, and thus from undergoing a temperature rise without a correct detection by the sensor.
Preferably, said welding is carried out through brazing. This advantageously allows preventing limestone deposits, as time passes, along the contact area between the heating source and the protective sheath and thus, a decrease in the sensitivity of the sensor.
Advantageously, said protective sheath has an elongated body. Typically, said sheath is a stainless-steel tube.
Typically, said heating source is a resistor.
Advantageously, the boiler also comprises a fuse. Preferably, said fuse is welded onto said elevated portion, in an opposed position with respect to said temperature sensor. The fuse is suitable to burn and to consequently switch off the heating source when it reaches a predetermined dangerous temperature (for example, equal to about 190xc2x0 C.). This allows protecting the apparatus of the invention from excessive rises of temperature of the heating sourcexe2x80x94due for example to a failure of the temperature sensor or of the water feeding meansxe2x80x94which may be dangerous.
Advantageously, the apparatus of the invention also comprises control means suitable to keep the level of water into the boiler at a predetermined value.
Preferably, said control means co-operate with said temperature sensor so as to drive said water feeding means so that they supply water to the boiler when said temperature sensor detects a temperature above a predetermined threshold temperature S1.
According to an embodiment, said boiler also comprises a pressure gauge suitable to detect the value of the steam pressure inside the boiler.
Advantageously, said control means are suitable to co-operate with said pressure gauge so as to switch said heating source on and off according to the pressure value measured by said pressure gauge, so as to keep the steam pressure into the boiler at a predetermined value.
Typically, said water feeding means from the reservoir to the boiler comprise an electrical micro-pump. Advantageously, said electrical micro-pump is of the vibrating type.
Preferably, at the start-up of the apparatus of the invention, said control means drive said water feeding means so that they supply a quantity of water to the boiler. More preferably, said control means drive said feeding means when the apparatus of the invention has been switched off for a predetermined period of time. This aspect of the invention is advantageous as it prevents the heating source from emerging from the water, thus overheating, during the start-up step, when the volume of water into the boiler is less than when in stand-by condition (which corresponds to the situation in which the pressure of the steam into the boiler has reached the desired value and the boiler is ready to deliver steam). In fact, at start-up, passing from an ambient temperature to a stand-by temperature (for example, of 130-140xc2x0 C.), the water of the boiler is subject to a volume expansion (generally, of at least 6%).
Furthermore, the above characteristic allows priming the electrical micro-pump before generating steam into the boiler. This is an advantage for vibrating pumps as these pumps may have priming problems when the boiler is already in pressure.
Advantageously, the water reservoir comprises a sensor suitable to detect the water level contained into it.
Preferably, when the level of water detected by said sensor is lower than a predetermined threshold value, said control means switch on a warning pilot lamp for the user, and switch off the water feeding means and the heating source. This advantageously allows warning the user on the need of filling the reservoir with water and preventing the water feeding means and the heating source from operating when the water into the reservoir is finishing.
Advantageously, when the level of water detected by said sensor is lower than said predetermined threshold value, said control means also provide to close said steam delivery means from the boiler to the user appliance. This allows keeping the boiler ready to deliver steam again preventing the user from continuing recalling steamxe2x80x94in case he does not notice the pilot lamp indicating the level of water into the reservoirxe2x80x94thus emptying the boiler. In fact, an emptying of the boiler would cause a delay in the restoration of the operating conditions of the apparatus, after filling the water reservoir, due to the time required by the boiler to be refilled with water, and to that required by the water to be re-vaporised at the desired conditions.
In a second aspect thereof, the present invention also relates to a heating unit, for a household apparatus for steam generation, comprising a heating source with an elevated portion which extends along a predetermined direction, and a temperature sensor contained into a protective sheath, said protective sheath being in contact with said heating source, characterised in that the contact area between said protective sheath and said elevated portion extends along said predetermined direction so as to make said contact area relatively wide.
As regards the characteristics of said heating source, of said temperature sensor, of said protective sheath and of said contact area, reference shall be made to what described above with reference to the apparatus of the invention.